1.) If you have the time to beat on a render with your computer, turn ambient off. This also means that you have to put in all the real light sources the scene will have. HDRI is a great way to crack that problem if the HDRI "backdrop" is what you want. If not, add all the lights in the right places and right colors and the scene will start to look real. 2.) If you don't have time to beat on an image, you can fake radiosity light sources by adding more lights to the scene. In the image I've attached, the first image is only raytracing, no ambient, no anything! The second image has five lights added for radiosity fake out. I did this in Raydream so that has to be true! The third image was done as at est in C2 with GI. Once again, no ambient. 3.) You can use ambient and it won't wreck the realism of an image but the application space where that can be done is limited (typically to outdoors and typically at a very low level). Just three points from my experience. To get back to the origin of the thread, I think I'd like to know specifically what it is about the images that suck. Scale of the model does play a big role in how the render works out (something that isn't obvious I might add!).

-Kix